This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2001-038023, filed Feb. 15, 2001, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a color cathode ray tube, and more particularly, to a color cathode ray tube in which the outer surface of an effective portion of a panel is substantially flat.
2. Description of the Related Art
For higher visibility, the outer surface of the effective portion of the panel of modern color cathode ray tubes tends to be flattened. As the outer surface of the effective portion becomes flatter, it is necessary to flatten the inner surface of the effective portion and the effective surface of a shadow mask that is opposed to a phosphor screen on the inner surface of the effective portion. Possibly, the effective surface of the shadow mask may be flattened by simply increasing the curvature radius of a conventional shadow mask that has a spherical effective surface. If high-density electron beams are emitted from an electron gun to display a high-intensity image, in this case, however, they run against the shadow mask, thereby causing the mask to undergo substantial local thermal expansion. Thus, the shadow mask bulges (or domes) toward the phosphor screen and causes lowering of color purity. Moreover, the curvature retention of the shadow mask may be lowered, so that the color cathode ray tubes can be deformed and rendered defective by impact or the like that acts thereon during their manufacture or transportation.
A panel is described in Jpn. Pat. Appln. KOKAI Publication No. 9-245685 as a measure to ease the above problems. The outer surface of the effective portion of this panel is flat, the major-axis-direction curvature radius of its inner surface is substantially infinite, and the minor-axis-direction curvature radius of the inner surface is substantially fixed. Described in Jpn. Pat. Appln. KOKAI Publication No. 10-199436, moreover, is a shadow mask that has an effective surface of the same configuration. Described in Jpn. Pat. Appln. KOKAI Publication No. 11-242940, furthermore, is a shadow mask of which the curvature is greater in the peripheral part of its effective surface.
The panel and each shadow mask described above have their respective specific effects against the aforesaid problems. In order to improve the visibility of the color cathode ray tube, however, the inner surface of the effective portion of the panel or the effective surface of the shadow mask must be further flattened. As for the shadow mask, moreover, it should be shaped so that it enjoys improved curvature retention.
The present invention has been contrived in consideration of these circumstances, and its object is to provide a color cathode ray tube with improved visibility, in which the effective surface of a shadow mask has high curvature retention.
In order to achieve the above object, a color cathode ray tube according to an aspect of the invention comprises: an envelope including a panel having a substantially rectangular effective portion, the effective portion including an outer surface in the form of a substantially flat surface or a curved surface having some curvature; a phosphor screen on the inner surface of the effective portion; an electron gun configured to emit electron beams toward the phosphor screen; and a shadow mask located between the phosphor screen and the electron gun, the shadow mask including a substantially rectangular effective surface opposed to the phosphor screen and formed having a number of electron beam passage apertures, the effective surface having a major axis extending at right angles to a tube axis and a minor axis extending at right angles to the tube axis and the major axis and being formed of a curved surface convexed on the phosphor screen side.
The effective surface of the shadow mask is formed so as to fulfill relations:
2xe2x89xa6RH1/RH2xe2x89xa64, and 
1 less than RV1/RV2xe2x89xa61.47, 
where H is the distance from the center of the effective surface of the shadow mask to the major-axis end portion of the effective surface, RH1 is an average major-axis-direction curvature radius of the effective surface on the major axis defined by a distance H/2 from the center of the effective surface to a midpoint in the major-axis direction and a fall ZH1 of the effective surface in the direction of the tube axis with respect to the center of the effective surface at the major-axis-direction midpoint, RH2 is an average major-axis-direction curvature radius on the major axis defined by a distance H and a fall ZH2 of the effective surface in the direction of the tube axis with respect to the center of the effective surface at the major-axis end portion, V is the distance from the center of the effective surface of the shadow mask to the minor-axis end portion of the effective surface, RV1 is an average minor-axis-direction curvature radius on the minor axis defined by a distance V/2 from the center of the effective surface to a midpoint in the minor-axis direction and a fall ZV1 in the direction of the tube axis with respect to the center of the effective surface at the minor-axis-direction midpoint, and RV2 is an average minor-axis-direction curvature radius on the minor axis defined by a distance V to the minor-axis end portion and a fall ZV2 in the direction of the tube axis with respect to the center of the effective surface at the minor-axis end portion.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.